Optical Rotation

Optical Rotation This is a property of the arrangement in space of the atoms in a compound, i.e. their stereochemistry.
	Normal light can be regarded as a transverse wave motion (vibrating at right angles to the direction of travel, like waves on water). When normal light, which has all possible directions of vibration, is passed through certain solids it becomes 'plane-polarized', that is, it vibrates at right angles to the direction of propagation in only one plane, instead of all possible planes. Optical rotation is measured with a polarimeter. It produces plane-polarized light and passes it through the sample of liquid and measures the angle through which the plane of vibration of the polarized light is rotated as it passes through the sample. The angle of specific rotation is called alpha and is expressed as either a positive or negative value depending on the direction of rotation. The optical rotation of an essential oil is a summation of the optical rotations of its chemical constuents, and is in relation to their proportions within that oil. For most oils a value within a specific range will be quoted owing to the variations in the natural composition. Any deviation from that range is a good indication of adulteration or impurity.